The invention relates to a bainitically hardened brake disk comprising at least one ring-shaped cast disk body with a radially extending ring section in contact with brake shoes mounted on a wheel hub together with a vehicle wheel for braking purposes.
Wheels of motor vehicles as a rule consist of a wheel body with a rim and base for the tire, and a dimensionally stable bowl which is centrally held to the flange of a wheel hub by bolts. The hub provides the bearing for the wheel and is itself mounted on the front or rear axle. A brake drum or disk of a disk brake assembly are detachably mounted on the wheel hub for braking the vehicle wheels. Disk brakes, which have now become accepted in modern cars, give a high braking effect while causing considerable wear. As a result, the worn brake disks of a motor vehicle need to be replaced several times during its lifetime. For easy replacement without dismantling the wheel hub, the same flange on the hub serves to fasten the wheel bowl and brake disk, requiring hat-shaped mounts and brake cups on the disk which account for a significant part of the weight of the brake assembly.
To facilitate the dismantling of disk brake assemblies, split brake disks have been proposed in which a worn friction ring can be separately replaced. DE 195 28 434 A1 describes a disk consisting of a friction ring, pin-shaped links and hubshaped bearing part. The links are held to the bearing part by a clamping ring and wheel bolts. The brake disk as a whole consists of the bearing part, an internally ventilated friction ring, and five links centered by a locating ring. As the links are made of different materials, not all of the advantages which would result from the use of castings manifest themselves.
As a solution to engineering problems encountered in manufacturing a brake disk described in DE 195 22 677 C1 which consists of a flat ring and is attached to the wheel hub, it is proposed to use an aluminum hub. This is again held by at least five bolts engaging recesses provided in the brake disk, said bolts being hollow to save weight. The bolts are knurled along part of their circumference and form-fitted into holes in the hub. It is well known that form-fitted links are highly sensitive to temperature and thus need to be protected from excessive heating. In an aluminum hub, the brake disk should therefore be mounted on the hub in a floating manner so that the heat generated by breaking may not be transferred from the brake disk to the hub. This type of floating arrangement is to guarantee optimal thermal insulation between the brake disk and wheel hub.
In view of the temperature sensitivity of aluminum, car manufacturers have made enormous efforts to replace aluminum components by gray iron castings. Of particular importance are the heat resistance properties of automotive components subjected to elevated temperatures. Here cast iron is superior to aluminum in that it has greater mechanical and fatigue strength at higher temperatures, and is highly resistant to hot cracking and more rigid while still offering good heat conductivity. Cast iron in particular provides good damping mainly because its microstructure contains graphite, an effect which also depends on the type of graphite interacting with the metallic matrix. This makes cast iron brake components particularly vibration damping and convenient.
Brake disks with flat separate friction rings made of cast iron for motor vehicle brakes may vary in construction. The publicly distributed printed copy of the application for DE 42 37 372 describes a brake disk consisting of at least three ring-shaped disk portions arranged side by side and joined to a pot-shaped disk carrier by intermittently arranged rivets. The outer disk portions are one-piece gray iron castings while the inner portions are laminated from at least two stacked sheet metal parts in order to dampen brake noise and squealing.
The publicly distributed printed copy of the application for DE 43 33 517 describes a brake disk consisting of a cast iron disk rim and form-fitted inner pot made of suitable steel sheet. The inner pot and disk rim are to be joined by laser welding without a filler thus giving a composite part with an inner pot characterized by higher tensile strength. According to the publicly distributed printed copy of the application for DE 195 05 112, the cast iron friction ring may be form-fitted in the direction of rotation with the pot-shaped holder provided for attachment to the vehicle wheel in such a way as to permit slight shifting parallel to the axis. The holder and friction ring have meandering indentations between which a meandering metal strip is inserted. During casting, the insert prevents the holder and friction ring from fusing together. This is to prevent warping of the disk from its plane as a result of braking or high temperatures, which may cause rubbing of the brake.
According to the patent specification for DE 34 32 501 C2, brake disks in rail vehicles exposed to even higher loads with a risk of cracking due to thermal stress are provided with radial slots at a uniform angular pitch to accommodate deformation from expansion during braking or thermal expansion. These arrangements may not suffice for disk brakes on high-speed trains where the large number and considerable weight of conventional brake disks made of gray and ductile iron, or cast steel, makes their use problematic. The solution proposed in the patent specification for DE 44 00 896 is a monobloc disk composed of a wear-resistant aluminum-silicon base alloy. This has the drawback of stiffening elements inserted into the cast structure which may increase production costs so that the approach is not applicable to conventional motor vehicle brake assemblies. Another disadvantage for use in road vehicles is less heat resistance at higher temperatures, for example when going downhill slowly with no cooling from the airflow.
For the above-mentioned reasons, high-strength cast iron grades have been suggested for use in heavy-duty vehicle brake disks. These are known as acicular ductile cast iron or, in the Anglo-Saxon terminology, as Austempered Ductile Iron (ADI). A well-known method for making ductile iron is to add small amounts of Mg to the melt, which produces nodular graphite to increase strength and toughness. Austempering has proven to be the most efficient procedure for further improving these properties.
In this connection, U.S. patent specification 5603 784 describes a method for producing brake drums and brake disk rings which are subject to premature wear from asbestos-free brake shoes and brake pads of disk brakes. It proposes rotatable brake components made of cast gray iron as particularly suitable for heavy duty trucks. The first step is providing a cast gray iron brake drum or disk with a carbon content between 3.4% and 4%. This is followed by an austempering heat treatment step which involves heating to between 816xc2x0 C. and 927xc2x0 C. and maintaining this temperature, and then quenching in a liquid bath at a temperature between 149xc2x0 C. and 371xc2x0 C. for 2-4 hours. The third step is retempering to provide a microstructure which consists of spheroidized pearlite carbon in a matrix of bainitic and austenitic iron. Brake disks made by this process have excellent wear resistance.
Brake disks made with a continuous bainitic or bainitic-austenitic microstructure throughout have the disadvantage of being less suitable for machining processes such as drilling, and a distortion caused by bainitic treatment. In addition, ductile iron does not have the desirable damping capacity of gray cast iron.
To avoid sound emission and vibration, the inventive brake disk is made of cast iron and has a high strength and low weight. The aim of the invention is particularly to avoid the disadvantages of combining a brake disk with a wheel hub made of aluminum.
According to the invention, the body of the brake disk is made of bainitically hardened gray cast iron (AGI) throughout. Bainitizing considerably improves the properties of gray cast iron, imparting very high strength, high wear resistance and an extraordinary damping capacity superior to that of a brake disk made of ductile iron. The microstructure of bainitic gray cast iron provides a wear resistance which is superior to that of high-strength gray cast iron, vermicular cast iron, and as-cast ductile iron. Even though these advantages have been listed for brake components by B. Kovacs in Konstruieren und Giexcex2en, vol. 19 (1994), no. 3, p. 16 ff., it has so far been impossible to use brake disks of this type. The reason is that their damping capacity derives from the high internal stress generated by small grain sizes and low bainitizing temperatures, which may lead to stress cracks from machining operations such as drilling or threading. This excludes a frictional connection between the brake disk and hub.
In order to overcome these disadvantages, it is further proposed to produce a one-piece brake disk by composite casting of the wheel hub and disk body with a material connection in the joining region, wherein the wheel hub material is cast iron, cast steel or formed steel of a higher tensile strength greater than 170 N/mm2. Composite casting provides a high-strength brake disk without the engineering problems caused by drilling and threading the hardened microstructure.
This enables the production of a high-strength and wear-resistant brake disk with a disk body whose service life is at least equal to, or longer, than that of the wheel hub.
Eliminating the brake pot may, in other words, considerably reduce the weight of the brake disk so that it may be designed to give a useful life which, supposing average wear, is exactly the same as that of the motor vehicle. Regarding maintenance of the brake assembly, considerable cost advantages would result for the vehicle owner as there would be no need for brake disk replacement. The construction of the brake assembly and the method of bearing for the wheel could be improved and optimized. Considerable advantages could result for specific types of motor vehicles whose annual mileage is low, such as city cars.
The composite casting process for a bainitically hardened brake disk involves a first step in which a ring-shaped raw disk is cast from a gray iron melt in a mold. This is followed by casting on a wheel hub made of ductile iron to produce a composite casting which is dressed, balanced, bainitically hardened and then finished by grinding the disk portion. In a further development of the procedure, casting on a raw disk to a steel sheet hub inserted into a mold produces a raw brake disk in which the hub, due to its different chemical composition and microstructure, is excluded from bainitic hardening and thus retains its high tensile strength and toughness compared with the disk body.